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CHAPTER 7 ITERATIVE PROCESSING The For Next loop is flexible. If you want, you can use the Step keyword to change the value used to increment the counter variable as the following shows. For I As Integer = 1 to 100 Step 10 ProgressBar1.Value = ProgressBar1.Value + 1 Next As you can see, this example increases the value of the ProgressBar control s Value property by 10 (10 percent) on each iteration. If you need to, you can decrement the value of the counter variable, as you see in the following. For this example to work effectively, you must set the ProgressBar control s value property to 100. For I As Integer = 100 DownTo 1 ProgressBar1.Value = ProgressBar1.Value - 1 Next Note Because of the manner is which REALbasic compiles code statements, the For Next, and For Each loops are typically more efficient than the Do Loop. Do Until, and While Wend loops in terms of resources consumed (memory and processor time) by your application. The For Each Loop The For Each loop is designed to automatically process each element stored in a single- dimension array. The syntax for the For Each loop is outlined in the following: For Each element [As DataType] In array statements Next Element is a variable used to represent an item stored in an array; DataType is optional. When used, it specifies the data type of the data stored in the array.
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194 CHAPTER 7 ITERATIVE PROCESSING the optional Step value you can change the incremental value. Finally, using the optional DownTo keyword, you can set up a For Next loop to decrement, rather than increment, the counter variable. If you define a For Next loop s counter variable within the loop using the optional DataType keyword, the variable is local to that loop, meaning when the loop terminates, the variable ceases to exist. If your application must be able to read the value of the counter variable outside of the loop, then you need to define the variable outside the loop. To see the For Next loop in action, look at the following example. For I As Integer = 1 to 100 ProgressBar1.Value = ProgressBar1.Value + 1 Next In this example, a For Next loop is used to demonstrate how to visually control the display of data using a ProgressBar control. Specifically, the loop iterates 100 times, incrementing the ProgressBar control s Value property by one on each iteration. Figure 7-5 demonstrates the effect the For Next loop has in this example. Figure 7-5. Using a For Next loop to visually control a ProgressBar control, as seen on Linux Note The ProgressBar control is commonly used to display a graphic representing the status of a process as it is being executed. By default, the ProgressBar control has an initial value of zero and a maximum value of 100. To indicate progress, all you have to do is to increment the ProgressBar control s Value property.
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CHAPTER 7 ITERATIVE PROCESSING Note The preceding example controlled when line breaks occurred in output by referencing an EndOfLine object. EndOfLine provides access to properties representing end-of-line markers on Macintosh, Windows, and Linux. You learn more about REALbasic classes later in Chapter 8. Figure 7-4 shows the output that is produced when this example is executed on a computer running Windows. Note, if the value of intCounter is greater than ten to start with, then the While Wend loop and all the statements contained within it are skipped. Figure 7-4. Using a While Wend loop to populate an EditField control, as seen on Windows The For Next Loop The For…Next loop is designed for situations in which you know exactly how many times a loop must execute. For Next loops use a variable to keep track of loop execution to determine when the loop should cease execution. The value of the loop variable, or counter, can be increased or decreased, as appropriate during loop execution. The following outlines the syntax of the For…Next loop. For counter [As DataType] = start To | DownTo end [Step Value] Statements Next [counter] Counter is a local variable used to control loop execution. DataType is an optional parameter that provides you with the capability to declare the counter will be within the loop. If DataType is omitted, you must remember to declare the counter variable elsewhere within your code statements. Start sets the beginning value of the counter variable. End sets its ending value (for example, the value, which when reached, terminates the loop s execution). Value is an optional keyword. When used, it specifies the value to be used to increment or decrement the value of the counter variable as the loop processes. By default, the counter variable is automatically incremented by one on each iteration of a For Next loop. However, by specifying
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192 CHAPTER 7 ITERATIVE PROCESSING Figure 7-3. Using a Do Loop to populate a PopupMenu, as seen on Mac OS X The While Wend Loop The While Wend loop provides the capability to execute one or more code statements, as long as an evaluated condition remains True. The following shows the syntax of the While Wend loop. While Condition Statements Wend Condition is a placeholder representing any valid Boolean expression, as the following example shows. Dim intCounter As Integer = 1 While intCounter <=10 EditField1.Text = EditField1. Text + Str(intCounter) + " " + EndOfLine intCounter = intCounter + 1 Wend In this example, a While Wend loop is used to populate an EditField control with ten numbers, each of which is displayed on a different line. The While Wend loop tests a condition at the beginning of the loop. The While Wend loop is similar to the Do Until loop, but it lacks the Do Until loop s flexibility. Specifically, the While Wend loop does not provide the option of moving the conditional text to the end of the loop. You do not see this form of loop used often in REALbasic programs.
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CHAPTER 7 ITERATIVE PROCESSING Note The ComboBox control is a space-saving control that, when clicked, displays a list of String values from which the user can make a selection. From the REALbasic IDE, you can add items to a ListBox by assigning data to its InitialValue property. Like the ListBox control, REALbasic also lets you programmatically add items to a ListBox control the AddRow method. In this example, if the value assigned to intCounterhad been set to 11 or greater at the start, the loop would have been skipped. This behavior occurs because the tested condition is checked at the beginning of the loop. The second way to use the Until keyword when setting up a Do Loop is to move the Boolean expression to the end of the loop. The following shows the syntax for this form of the Do…Loop. Do Statements Loop Until condition The result of moving the tested condition to the end of the loop is this: the Do Loop executes once, no matter what the value of intCounter might be. Otherwise, this form of the Do Loop works pretty much the same as the first form, as the following example shows. Dim intCounter As Integer = 1 Do PopupMenu1.AddRow Str(intCounter) intCounter = intCounter + 1 Loop Until intCounter > 10 In this example, the Do Loop runs one time no matter what. However, because the initial value assigned to intCounter is one, it runs ten times, producing the same output as the previous example. Figure 7-3 shows the output produced when this example is executed on a computer running Mac OS X. Note You might have noticed in the last example that, instead of populating data to a ComboBox control, the data was added to a PopupMenu control. The two controls operate in a similar fashion. The main difference is that, in addition to enabling the user to select from a list of items, the ComboBox also provides the capability to accept input keyed in by the user.
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190 CHAPTER 7 ITERATIVE PROCESSING Using the Until keyword, you can set up two different forms of the Do Loop. The following shows the syntax for the first form. Do Until condition Statements Loop Condition is a Boolean expression that must evaluate to True for the loop to stop executing. Statements is a placeholder that represents the code statements you want executed when the loop executes. To see how this form of the Do Loop works, consider the following example. Dim intCounter As Integer = 1 Do Until intCounter > 10 ComboBox1.AddRow Str(intCounter) intCounter = intCounter + 1 Loop This example begins by declaring a variable named intCounter with a data type of integer and an initial value of one. Next, a Do Loop is set up that loads a list of numbers (1 10) into a ComboBox control, using the control s AddRow method. Because the initial value of intCounter is equal to one and is incremented by one on each iteration of the loop, the loop executes ten times before terminating its own execution. Figure 7-2 shows the output produced when this example is executed on a computer running Linux. Figure 7-2. Using a Do Loop to populate a ComboBox, as seen on Linux
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CHAPTER 7 ITERATIVE PROCESSING Note The ListBox control displays a list of String values from which the user can make a selection. You can add items to a ListBox from the REALbasic IDE by assigning data to its InitialValue property. REALbasic also lets you programmatically add items to a ListBox control using its AddRow method. Because the Do Loop does not specify a predefined condition that must be met for the loop to stop executing, it is up to the programmer to develop a means for stopping loop execution. In the previous example, this occurred when the value of intCounter was set equal to ten, at which time the Exit statement was executed. The Exit statement causes the immediate termination of a loop, allowing your application to continue processing, starting with the first code statement following the loop. Figure 7-1 shows the output produced when this example is executed on a computer running Windows. Figure 7-1. Using a Do Loop to populate a ListBox, as seen on Windows You should avoid using the Do Loop and the Exit statement when creating loops within your REALbasic applications. Because the termination condition is not clearly identified at the beginning or the ending of the loop, the condition that will terminate loop execution is not readily evident. This is generally considered a poor programming practice because it forces you to read through all the code statements contained within the loop to identify how it is terminated. A better way of setting up a Do Loop is to include the optional Until keyword. This enables you to set up loops that execute until a specific condition becomes True or, to put it another way, the Do Loop executes as long as a specified condition remains False. This form of the Do Loop is perfect for situations where you know in advance what condition must be met for the loop to terminate its execution. For example, you might set up a Do Loop to run until a variable was set equal to a certain value or until the user entered a certain command.
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188 CHAPTER 7 ITERATIVE PROCESSING The following list identifies the different types of loops that REALbasic supports: Do…Loop. Creates a loop that iterates until terminated by an Exit statement. For…Next. Creates a loop that iterates a set number of times. For Each. Creates a loop that iterates through every element stored in an array. While Wend. Creates a loop that iterates as long as a specified Boolean condition remains True. Each of these different types of loops is examined further in the following sections. In addition, this chapter also teaches you how to work with a number of new controls as you work your way through the following loop examples. Do Loop The Do Loop is the simplest type of loop supported by REALbasic. It supports a number of different variations. In its simplest form, the Do Loop executes without checking for a predefined condition, using the following syntax. Do Statements Loop Do is a keyword identifying the beginning of the loop. Statements is a placeholder representing code statements you embed inside the loop for repeated execution. Loop is a keyword that identifies the end of the loop. To get a good idea of how to work with this format or the Do Loop, consider the following example. Dim intCounter As Integer = 0 Do intCounter = intCounter + 1 ListBox1.AddRow Str(intCounter) If intCounter = 10 Then Exit End If Loop This example begins by declaring a variable named intCounter with a data type of Integer and an initial value of zero. Next, a Do Loop is set up that loads a list of numbers (1 10) into a ListBox control. The Do Loop increments the value of counter by 1 each time the loop iterates. The ListBox control s AddRowmethod is used to add a number to the ListBox control. Note, the number added is stored in the intCounter variable. This variable has a data type of integer, so it must be converted to a string before it can be added to the ListBox control.
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CHAPTER 7 Iterative Processing In Chapter 6, you learned how to set up conditional tests that control the logical flow of program code within your REALbasic applications. In this chapter, you learn how to set up loops to process large amounts of data or to repeatedly execute a set of statements over and over again. REALbasic provides support for a number of different types of loops and you learn how to work with each type. In addition, you also learn how to work with a number of new controls, such as the ListBox, ComboBox, PopupMenu, ProgressBar, and MoviePlayer controls. Specifically, this chapter teaches you how to Set up and control different types of loops Programmatically break out of endless loops Terminate endless loops from within the REALbasic IDE Work with the ListBox, ProgressBar, PopupMenu, GroupBox, and MoviePlayer controls Processing Data with Loops Any time you need to process large amounts of data or to perform a series of code statements over and over again, you need set up a loop. A loop is just a series of statements, processed as a block, that execute repeatedly until a certain condition is met. For example, you could create a loop that collects user input until the user clicks a Finished button. You could also set up a loop to open a file and read every line in the file until the end of the file is reached. Or, you could set up a loop that iterates until a given variable is set to a certain value. The number of cases where loops come in handy is endless. By enabling you to process the same code statements over and over again, loops help to streamline program code by letting you process any amount of data by reusing the same set of code statements. Loops are ideal for driving any repetitive tasks and for processing the contents of arrays. You see examples of how to perform both of these types of tasks in this chapter. While, in most cases, you can use any one of the types of loops supported in REALbasic to perform a given task, each of these loops has certain qualities that differentiate them and make them better suited to particular types of tasks.
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186 CHAPTER 6 MAKING DECISIONS WITH CONDITIONAL LOGIC Figure 6-7. By adding an About menu item to your menu system, you provide a place for copyright or other useful application information. If you don t run across any errors when you test the RB Number Guess game, go ahead and compile standalone copies of the application. If you do run into errors when testing the application, they are most likely the result of typos you made when keying in program code statements. Go back and review your program code, and then fix any typos you find. Summary In this chapter, you learned the ins and outs of applying conditional logic to your REALbasic applications to alter the logical flow of program code. This included learning how to work with the many formats of the If Then, as well as with the Select Case statements. You learned how to use #If #EndIf to create conditional logic that executes based on the OS or version of REAL- basic running it. You also learned how to work with REALbasic s mathematical, comparison, and logical operators. On top of all this, you had the chance to add custom window properties, and you learned how to generate random numbers and how to work with the Slider control.
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